Andy i do this. I clean at one site and we do not use any water tanks etc. we use the customers external taps to feed the power washer.
Have two machines, only use one and one is kept as a spare incase there's a problem. The machines are a small Clarke Tiger 2500 and a Chinese sub £200 ebay jobby. I'm not recommending them as machines to use i just use them because i had them. Them both have a chemical pick up for applying TFR. Mostly they still need a scrub with a brush, we use one of them small "caravan " cleaning brushes for the low level stuff up to say head height and an old WFP brush with a soft, rounded Vikan brush for the tops and windscreens etc.
Also we only clean the tractor units and no trailers or box bodies.